Nitrous oxide (N2O) is a greenhouse gas with a global warming potential of 310 times that of CO2 and an atmospheric lifetime of 114 years. Automotive exhaust is one possible source of N2O emissions, as a by-product of combustion of fuel itself as well as a by-product formed during the catalytic reduction of NOx. Recognizing its global warming potential, US EPA has already set a N2O emission limit of 10 mg/mile for light-duty vehicles over the FTP cycle starting from MY2012, and a N2O emission limit of 0.1 g/bhp-h for heavy duty vehicles over the heavy duty FTP cycle starting from MY2014. In the past, automobile catalyst systems were normally optimized for maximum reduction of NOx (a regulated pollutant) without accounting for N2O level. Now if N2O exceeds the 10 mg/mile limits, then there is a penalty against CAFE fuel economy requirements.
Currently, nitrous oxide (N2O) decomposition is practiced industrially for treating the off-gases from nitric acid and adipic acid production. The temperatures for these operations are much higher (>550° C., for example ˜800-900° C.) than that of typical automotive exhaust, and the process streams contain little water (<1%). There are many literature reports describing N2O decomposition catalysts, and most can be grouped into three categories: (1) supported Rh, (2) metal oxides with spinel structure and (3) ion exchanged zeolites. Such catalysts are usually in powder or pelleted form and not supported on a ceramic carrier, such as a monolithic substrate or a wall-flow filter. In DE102008048159, decomposition of N2O in a gas current is conducted with a catalyst where rhodium is supported on a gamma-alumina that is optionally doped with cerium or gold.
In KR20060019035, directed to a method for removing nitrogen oxides by using dual catalyst beds, nitrogen oxides are decomposed into nitrogen and nitrous oxide using a bed of nitrogen oxide reducing catalyst Pt/VX-PY-(material containing hydroxyl group)z, and the nitrous oxide is further decomposed into nitrogen and oxide using a bed of nitrous oxide decomposing catalyst Rh—Ag/CeO2/M1-M2-M3, where M1 is Mg, Ba or Sr, M2 is Al, Fe, V, Ga or Cr, and M3 is Zn, Ni, Cu.
WO2011036320 is directed to catalytic systems of rhodium and cerium oxide comprising an active rhodium phase supported on a mixed oxide of cerium and one or more metals selected from transition and internal transition metal groups, and a support such as alumina. WO2011036320 targets effluents that are characterized by containing diluted nitrous oxide (typically 500-5000 ppm), relatively low temperature (<525° C.), and inhibitor gases.
In U.S. Pat. No. 8,512,658 a method of depleting nitrous oxide in exhaust gas after-treatment for lean-burn internal combustion engines is provided. A N2O depletion catalyst is preferably a catalyst selected from the group consisting of a three-way catalyst, a NOx reduction catalyst, a NOx storage catalyst and an oxidation catalyst. U.S. Pat. No. 8,512,658 identifies a particular embodiment of a N2O depletion catalyst as being palladium supported on a high-surface-area metal oxide, preferably a lanthanum-stabilized aluminum oxide due to such a catalyst providing the lowest light off temperatures for the N2O reaction under λ≤1 conditions after aging. When the N2O depletion catalyst is below its light-off temperature, U.S. Pat. No. 8,512,658 identifies that it is advantageous to heat the catalyst.
There is a continuing need in the art to provide catalytic articles that efficiently and effectively provide removal of nitrous oxide (N2O) under exhaust gas conditions.